I'm not sure that physics really has anything to say about this question, unless someone happens to know of a physical effect that explains the phenomenon... let's see what others have to say.
–
David Z♦Jan 14 '11 at 0:45

4

Won't it just be the vibrations of the laptop? Namely the hard drive. try putting something thin which isn't an electrical conductor (Eg paper) between her and the laptop)
–
Jonathan.Jan 14 '11 at 1:43

1

Maybe the question could be restated in a manner that doesn't reference any particular individual, as in - "If a person uses a laptop in certain way, would that be responsible for a buzzing sensation on their skin?"
–
user346Jan 14 '11 at 4:30

1

the buzz is permanent until she breaks contact with the laptop
–
Tim GradwellJan 15 '11 at 9:18

6 Answers
6

I felt this and even drew sparks from it on a product I was working on, and was very worried, but after a lot of research and the standard government safety testing, it was deemed normal.

A small amount of touch current is allowed because it is not considered harmful. It happens with devices which have isolated transformers but the primary side is not grounded. There is a small amount of capacitance across the transformer isolation (especially in switch-mode power supplies, which require extra capacitance here to prevent EMI), which means the chassis will be at line voltage relative to Earth if you measure it with a voltmeter, but the capacitance is small so no significant current can flow.

If you concentrate the current to a very small point (by brushing two metal surfaces together, so that microscopic spikes touch each other momentarily), the metal will melt and make sparks. If you press the metal tightly together, nothing will happen.

If you ground yourself and then brush against the metal very lightly with a finger, especially on a corner or point, you will feel it, because it is high voltage and stimulating a few nerve cells at the tiny point of contact, but if you press hard you will not, because even though more nerve cells are in contact, your body's resistance/capacitance is grounding out the object and the voltage drops below the threshold of sensation. I also noticed that if I rubbed a shielded object against my skin (the shield of a cable), the friction sensation felt different with the power on and power off.

It sounds likely to me, but unfortunately we can't check the laptop without power mains because the battery no longer works. Next time I have access to an oscilloscope I will try this out.
–
Tim GradwellJan 15 '11 at 9:23

I found this phenomen touching aluminium surfaces of my external hard drive and some laptops. It goes if you pull the main plug and use battery.
–
MarcelFeb 2 '11 at 7:57

Its due to residual transductance of the live AC current into the shielding of the device. Get an earthed plug and it will disappear. I've experienced the issue in dozens of shielded but unearthed electrical appliances, not just laptops. It feels exactly like a vibration (when you move the finger over the surface) but it's not, my guess is that the mechanoreceptive nerve-endings in the skin react to the small current.

Having an "earthed plug" is not enough. There has to be a connection from the PE of mains througout the mains adaptor to the laptop case. This is not installed easily in an existing devioce. Sometimes it helps to rotate the mains plug for 180 degrees.
–
GeorgNov 18 '11 at 9:49

1 - Use a voltage detector pen in USB connector GND to see if there is a bad isolation between AC ground and DC ground (this is first option not because probability, but because it could be dangerous). It's normal a little fugue current between primary and secondary of transformer but the AC plug have 3 conectors, 2 for voltage and the third is for GND (ground), perhaps your home installation haven't a good grounding, check that! is very important.

2 - Beware of what is on your wife's laptop screen when her skin buzz =P

So I've been feeling the same thing with my girlfriend. I believe it has more to do with what Peer is talking about. Both my girlfriend and I are on our laptops, both plugged into power sources. She has a dell, I have a mac. At first I thought I could be that our comps are spinning at different rates, until I read this blog and experimented.

I touched her arm while we both had our comps on our laps and plugged into power source.
-vibration

I took my comp off my lap while she still had hers on her lap, then touched her arm.
-no vibration

I put my comp back on my lap and unplugged it from power source, she had hers on her lap still plugged into power source and I touched her arm.
-no vibration

I agree with jonathon that it is vibration, although it is far more likely to be the laptop fan, rather than the hard drive

Why she doesn't feel it? The human body has vibration receptors, and I'm sure she notices the laptop vibrating when she turns it on. However, like with all constant stimuli, but particularly touch, our brains are great at blocking them out. I suspect that the fact it happens to her whole body makes the effect even greater, as there is no sensory discrimination from different nerves

You touch her, and two things are different. For you, it is a new sensation, and it is occurring in your hand alone. If you held her for a while, while focussing on something else, you would probably stop noticing

I would argue that this is a physical effect, biophysical to be precise.